Recently, in order to inspect the processed and manufactured state of a fine structure having a complicated stepped structure due to miniaturization and refinement of electronic and mechanical parts, a high measurement accuracy to size, shape, and surface roughness has been required.
Therefore, a size measuring method using an optical two-dimensional measuring device and a shape or a thickness (surface roughness) measuring method using an optical three-dimensional measuring device are used now in measurement of small-sized electronic and mechanical parts.
As one of conventional optical three-dimensional measuring devices, a three-dimensional shape measuring apparatus using interferometry has been proposed.
FIG. 5 is a view illustrating the measurement principle of general interferometry. When illumination light emitted from a light source is split into beams by a beam splitter and the beams are respectively irradiated onto a reference mirror and the surface of a target object to be measured, a reference beam and an object beam respectively reflected by the reference mirror and the surface of the target object are joined by the beam splitter and interfere with each other to generate an interference pattern. Here, the interference pattern is formed at a point, where the beams irradiated respectively onto the surface of the target object and the surface of the reference mirror, i.e., a reference surface, have the same focal distance, and the optical paths of the reference beam and the object beam coincide with each other.
The height of the target object is measured by calculating the phase of the interference pattern by detecting the interference pattern with a light detecting element, such as a CCD camera, or abstracting a point with the maximum coherency from the envelope of the interference pattern.
Therefore, after an interference pattern obtaining area of a target object having a stepped structure is divided into uniform sections according to the height data, the reference surface or the target object is minutely moved according to the divided sections to generate interference patterns, and the obtained plural interference patterns are joined to measure the surface shape of the target object.
In case of a ball grid array (BGA) having a three-dimensional shape with a stepped structure, it is possible to infer the surface shape of the BGA or determine whether or not the BGA is defective by obtaining only interference patterns for the lowest point and the highest point.
However, even in this case, since the overall three-dimensional shape of the BGA cannot be measured from an image, which is obtained once, a single interference pattern is obtained by joining the interference pattern corresponding to the highest point and the interference pattern corresponding to the lowest point, which are respectively obtained, or an interference pattern for the overall regions from the highest point to the lowest point is obtained, and thus an inspection speed is low.
Further, in case that reflectivities at the highest and lowest points are different, for example, in case that the reflectivity at the highest point of the BGA made of a metal is high and the reflectivity at the lowest point of the BGA made of a material for a PCB is low, when the reflectivity of the reference surface coincides with the reflectivity of any one of the highest point and the lowest point, the other one of the highest point and the lowest point cannot be measured well.
In order to solve the above problems, Korean Patent Application No. 2007-0052290 entitled “Apparatus for measurement of three-dimensional shape”filed by the applicant of the present invention is disclosed.
The above three-dimensional shape measuring apparatus, as shown in FIG. 6, includes a light source 100, a beam splitter 200 to split illumination light emitted from the light source 100 into beams, a target object 300 to be measured, onto which the illumination light from the beam splitter 200 is irradiated and has a height difference between the highest point and the lowest point, a reference mirror 400, onto which the illumination light from the beam splitter 200 is irradiated, a light detecting element 500 to capture an interference pattern generated by joining beams respectively reflected by the surface of the target object 300 and the surface of the reference mirror 400, a reflection distance adjusting unit 700 to provide reflection distances, which are respectively equal to the reflection distance of the highest point of the target object 300 and the reflection distance of the lowest point of the target object 300, and a control computer 600 to process the image captured by the light detecting element 500.
Here, the reflection distance adjusting unit 700 has a thickness, which is equal to the height difference between the highest point and the lowest point of the target object 300, to provide reference beams A1 and A2 having reflection distances being equal to those of the highest point and the lowest point of the target object 300, i.e., having reflection distances being equal to those of object beams A1 and A2. Further, in case that the target object 300 is replaced with a new one and thus a height difference is changed, the reflection distance adjusting unit 700 adjusts reflection distances by adjusting a position according to the target object. Thereby, the reflection distance adjusting unit 700 is capable of generating reference beams being equal to object beams in connection with the target object to be measured.
Therefore, the above apparatus simultaneously obtains interference patterns for the highest point and the lowest point of the target object, and thus improves an inspection speed.
However, since an interference pattern is generated when focusing conditions are satisfied and reflection optical paths of the reference beam and the object beam coincide with each other, in case that height data of a target object to be measured are changed, when the reflection distances are adjusted, the focal distance of the beams does not coincide with each other, and when the focal distances are adjusted, and the reflection distances are do not coincide with each other.